In the cascade activation of glycogen utilization leading to energy production in mammalian skeletal muscle, phosphorylase kinase (PhK) phosphorylates and activates glycogen phosphorylase, which then degrades glycogen. The ( )4 PhK complex is among the largest and most complex enzymes known, with 90% of its 1.3 x 106 Da mass involved in its regulation (i.e., the integration of activating signals). The activity of PhK, catalyzed by its subunit, is markedly enhanced by multiple metabolic, hormonal and neural stimuli, which it integrates through allosteric sites on its three regulatory subunits, , and . Ca2+ is an obligatory activator, in that PhK's other activators have no effect in its absence, but further activate in its presence. The simultaneous activation of PhK by diverse biological signals allows for the tight control of glycogenolysis and subsequent energy production;and in skeletal muscle, activation of PhK by Ca2+ couples contraction with energy production to sustain contraction. Moreover, a regulatory region of PhK's catalytic subunit shares high sequence identity with the key functional region of the troponin-actin Ca2+-dependent regulatory system of muscle contraction, suggesting co-evolution of the regulation of contraction and energy production from glycogen. Our long term goal is to elucidate the changes in intersubunit interactions that lead to activation of PhK in response to different biological signals and the interplay between changes induced by the obligatory Ca2+ versus PhK's other activators. The proposed project consists of two broad aims related to the effects of activators on PhK. (1) We will characterize activator induced structural changes in PhK from the subunit level to the overall complex by employing a large variety of chemical, biochemical and biophysical techniques. (2) Through baculovirus (BV)-mediated expression of recombinant complexes, we will evaluate the function of residues and regions that are thought to participate in PhK's communication network. This will mark the first ever direct structure-function studies on this massive enzyme complex. The BV-expression system will also allow elucidation of the underlying reasons for human glycogen storage diseases caused by single missense mutations in PhK's large and regulatory subunits, which in the case of can lead to different phenotypes depending on where in the subunit the mutation occurs.